A fundamental problem in biology is to understand how seed development. In flowering plants, the ovule generates the female gametophyte, which is composed of egg, central, synergid and antipodal cells (Reiser, et al., Plant Cell, 1291-1301 (1993)). All are haploid except the central cell which contains two daughter nuclei that fuse prior to fertilization. One sperm nucleus fertilizes the egg to form the zygote, whereas another sperm nucleus fuses with the diploid central cell nucleus to form the triploid endosperm nucleus (van Went, et al., Embryology of Angiosperms, pp. 273-318 (1984)). The two fertilization products undergo distinct patterns of development. In Arabidopsis, the embryo passes through a series of stages that have been defined morphologically as preglobular, globular, heart, cotyledon and maturation (Goldberg, R. B., et al., Science (1994) 266: 605-614; Mansfield, S. G., et al, Arabidopsis: An Atlas of Morphology and Development, pp. 367-383 (1994)). The primary endosperm nucleus undergoes a series of mitotic divisions to produce nuclei that migrate into the expanding central cell (Mansfield, S. G., et al., Arab Inf Serv 27: 53-64 (1990); Webb, M. C., et al., Planta 184:187-195 (1991)). Cytokinesis sequesters endosperm cytoplasm and nuclei into discrete cells (Mansfield, S. G., et al., Arab Inf Serv 27:65-72 (1990)) that produce storage proteins, starch, and lipids which support embryo growth (Lopes, M. A. et al., Plant Cell 5:1383-1399 (1993)). Fertilization also activates development of the integument cell layers of the ovule that become the seed coat, and induces the ovary to grow and form the fruit, or silique, in Arabidopsis.
Of particular interest are recent discoveries of genes that control seed, and in particular endosperm, development. For instance, MEDEA (MEA) (also known as FIEI (see, e.g., copending U.S. patent application Ser. No. 09/071,838) and F644 (see, e.g., Kiyosue T, et al. (1999) Proc Natl Acad Sci USA 96(7):4186-91) encodes an Arabidopsis SET domain polycomb protein that appears to play a role in endosperm development. Inheritance of a maternal loss-of-function mea allele results in embryo abortion and prolonged endosperm production, irrespective of the genotype of the paternal allele. Thus, only the maternal wild-type MEA allele is required for proper embryo, endosperm, and seed coat development (Kinoshita T, et al. (1999) Plant Cell 10:1945-52). These results reveal functions for plant polycomb proteins in the suppression of central cell proliferation and endosperm development (Kiyosue T, et al. supra).
Another gene product that controls seed development is FIE, also known as FIE3 (see, e.g., copending U.S. patent application Ser. No. 09/071,838). The FIE protein is a homolog of the WD motif-containing Polycomb proteins from Drosophila and mammals (Ohad, N. et al. Plant Cell 11(3):407-16 (1999)). In Drosophila, these proteins function as repressors of homeotic genes. Loss of function mutations in the FIE gene result in endosperm phenotypes that are identical to medea loss of function mutations. A female gametophyte with a loss-of-function allele of fie undergoes replication of the central cell nucleus and initiates endosperm development without fertilization. These results suggest that the FIE Polycomb protein functions to suppress a critical aspect of early plant reproduction, namely, endosperm development, until fertilization occurs. Moreover, hypomethylation of fie mutants leads to the development of differentiated endosperm. Vinkenoog et al., Plant Cell 12:2271-2282 (2000).
Control of the expression of genes that control egg and central cell differentiation, or those that control reproductive development, i.e. embryo, endosperm and seed coat, is useful in the production of plants with a range of desired traits. These and other advantages are provided by the present application.